1. Field of the Invention
The invention relates to a drilling tool with a replaceable bit particularly for bores with depths greater than four times the diameter of the bore to be made, the drilling tool being formed in a drill shaft preferably with an integrated chuck shaft for fitting in a machine tool, and with a drill bit to be attached to the drill shaft.
2. Description of the Prior Art
A tool of this type is known from practice, which is suitable for bore depths of up to eight times the bore diameter, and a pilot drill being located centrally in a drilling head. This pilot drill is supplemented on the drilling head by indexable inserts screwed onto the drilling head, of which at least one sits internally in the vicinity of the pilot drill and a further insert sits externally, and thus produces the effective bore diameter. Such drilling heads have a head surface with multiple teeth on the shaft side, which are inserted into complementarily designed projections or returns on the drill shaft, and connect the drilling head with the drill shaft by screws lying in the axial direction. The pilot drill sits in a bore centrally located in the drilling head and penetrating through said drilling head, and extends into a central blind bore in the drill shaft. The pilot drill is secured in the drill shaft by a clamp screw located transversely to the drilling axis, and which engages into a corresponding recess on the outer circumference of the pilot drill. In addition, the axis of the pilot drill, which may also be provided with a central cooling duct, can be tilted in the axial direction by means of a set screw to be inserted orthogonally to the axis of the pilot drill laterally in the drill shaft.
Both triangular and quadrangular plates of hard metal are used as indexable inserts.
In a similar embodiment, the pilot drill and the indexable inserts are secured directly at the drill point by clamp screws or clamp claws located transversely to the axis.
In such drills, which are available for a diameter range of 20 to 50 mm or above, problems arise with setting up such drill points due to the asymmetrical cutting conditions, as the pilot drill must be set up precisely with relation to the drilling geometry and the metal to be drilled. The cutting edges of the drills are not continuously distributed over the radial circumference, and the cutter blades lie a small extent behind the point of the pilot drill. This gives rise to considerable problems when initially drilling workpieces provided with rotary centering means or during initial drilling of oblique surfaces which are inclined through more than 4 to 8.degree.. In addition these drills are not suitable for drilling up pre-drilled workpieces, as no centering of the drilling tool relative to the centre of the axis of the bore to be produced is possible. During packet drilling there always arises at the bore outlet a small shoulder, as the pilot drill is already out of cut at this point and the asymmetrical blades push away the material to be machined on one side. For reasons of constructions and material technology such drills also cannot be produced for bore diameters below 20 mm and at the smaller diameters, there is the problem of build-up of chips in the centre of the axis.
On the other hand, the use of normal HSS drills for diameters below 20 mm is uneconomic for many fields of application, particularly with high feed speed and a high cutting speed, particularly for "smearing" materials and large bore depths, as after a short time, due to lack of suitable cooling, the cutting surfaces and guide surfaces overheat and must be reground.
There is known from DE 37 09 878 C2 an HSS drill with a replaceable bit, in which the bit, provided with two complete radial cutters, is placed with its shaft-side surface in a correspondingly formed centering of the head surface of a drill shaft and is connected by means of two threaded pins located parallel to the drill axis, with the drill shaft. The drill can also comprise cooling ducts. Such a constructive type cannot be produced for drill bore diameters below 12 mm, and due to the outlay on time, replacement of the bit is more expensive than the drill.
There is known from DE 33 06 209 C2 a replaceable hard metal insert with a double cutter for smaller-diameter drills. In this case a cutter plate with shaft-side centering pins can be inserted into the centre of the drill shaft in a correspondingly prepared drill shaft, from a specially milled-out chip groove; said cutter plate can be pivoted contrary to the rotary working direction of the drill in a plate seat in front of a drive means on the shaft. Thus the shaft-side surface of the cutter plate can be clamped against a head surface of the plate seat on the drill shaft, the centering pin, during the pivoting movement, executing with its conical peripheral surface a spiralling movement towards the shaft side, thus drawing the cutter plate along with it.
This type of attachment generates tensions in the hard metal plate which are concentrated in the throat between pin and plate as superimposed bending and torsion forces. When loads are applied during machining, the hard metal insert then becomes liable to breakage at this point. Therefore this arrangement has not become commonly used in practice.